Current implantable medical devices for cardiac applications, such as pacemakers, include a “housing” or “can” and one or more electrically-conductive leads that connect to the can through an electro-mechanical connection. The can is implanted outside of the heart, in the pectoral region of a patient and contains electronics (e.g., a power source, microprocessor, capacitors, etc.) that provide pacemaker functionality. The leads traverse blood vessels between the can and heart chambers in order to position one or more electrodes carried by the leads within the heart, thereby allowing the device electronics to electrically excite or pace cardiac tissue and measure or sense myocardial electrical activity.
To sense atrial cardiac signals and to provide right atrial chamber stimulation therapy, the can is coupled to an implantable right atrial lead including at least one atrial tip electrode that typically is implanted in the patient's right atrial appendage. The right atrial lead may also include an atrial ring electrode to allow bipolar stimulation or sensing in combination with the atrial tip electrode.
Before implantation of the can into a subcutaneous pocket of the patient, however, an external pacing and measuring device known as a pacing system analyzer (PSA) is used to ensure adequate lead placement, maintain basic cardiac functions, and evaluate pacing parameters for an initial programming of the device. In other words, a PSA is a system analyzer that is used to test an implantable device, such as an implantable pacemaker.
To sense the left atrial and left ventricular cardiac signals and to provide left-chamber stimulation therapy, the can is coupled to the “coronary sinus” lead designed for placement in the “coronary sinus region” via the coronary sinus ostium in order to place a distal electrode adjacent to the left ventricle and additional electrode(s) adjacent to the left atrium. As used herein, the phrase “coronary sinus region” refers to the venous vasculature of the left ventricle, including any portion of the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other cardiac vein accessible by the coronary sinus.
Accordingly, the coronary sinus lead is designed to: receive atrial and/or ventricular cardiac signals; deliver left ventricular pacing therapy using at least one left ventricular tip electrode for unipolar configurations or in combination with left ventricular ring electrode for bipolar configurations; deliver left atrial pacing therapy using at least one left atrial ring electrode as well as shocking therapy using at least one left atrial coil electrode.
To sense right atrial and right ventricular cardiac signals and to provide right-chamber stimulation therapy, the can is coupled to an implantable right ventricular lead including a right ventricular (RV) tip electrode, a right ventricular ring electrode, a right ventricular coil electrode, a superior vena cava (SVC) coil electrode, and so on. Typically, the right ventricular lead is inserted transvenously into the heart so as to place the right ventricular tip electrode in the right ventricular apex such that the RV coil electrode is positioned in the right ventricle and the SVC coil electrode will be positioned in the right atrium and/or superior vena cava. Accordingly, the right ventricular lead is capable of receiving cardiac signals, and delivering stimulation in the form of pacing and shock therapy to the right ventricle.
Although a portion of the leads are located within the heart, a substantial portion of the leads, as well as the can itself are outside of the patient's heart. Consequently, bacteria and the like may be introduced into the patient's heart through the leads, as well as the can, thereby increasing the risk of infection within the heart. Additionally, because the can is outside of the heart, the patient may be susceptible to Twiddler's syndrome, which is a condition caused by the shape and weight of the can itself. Twiddler's syndrome is typically characterized by a subconscious, inadvertent, or deliberate rotation of the can within the subcutaneous pocket formed in the patient. In one example, a lead may retract and begin to wrap around the can. Also, leads may dislodge from the endocardium and cause the device to malfunction. Further, in another typical symptom of Twiddler's syndrome, the device may stimulate the diaphragm, vagus, or phrenic nerve, pectoral muscles, or brachial plexus. Overall, Twiddler's syndrome may result in sudden cardiac arrest due to conduction disturbances related to the device.
In addition to the foregoing complications, implanted leads may experience certain further complications, such as incidences of venous stenosis or thrombosis, device-related endocarditis, lead perforation of the tricuspid valve and concomitant tricuspid stenosis; and lacerations of the right atrium, superior vena cava, and innominate vein or pulmonary embolization of electrode fragments during lead extraction.
To combat the foregoing limitations and complications, small sized devices configured for intra-cardiac implant have been proposed. These devices, termed leadless pacemakers (LLPM) are typically characterized by the following features: they are devoid of leads that pass out of the heart to another component, such as a pacemaker can outside of the heart; they include electrodes that are affixed directly to the can of the device; the entire device is attached to the heart; and the device is capable of pacing and sensing in the chamber of the heart where it is implanted.
It can be appreciated, however, that a leadless pacing system needs to be compact enough to fit within the heart. At the same time, the pacing system requires a power source to operate. Accordingly, the pacer module includes a battery contained therein. Typically, the pacer module has a housing having a battery that may take up as much as 75% of the internal volume of the housing. Therefore, the pacer module itself may be bulky and occupy a relatively large volume within the heart chamber, which may adversely impact proper heart function.
Moreover, many pacing systems include telemetric subsystems that communicate with a remote programmer or patient care system. Telemetric subsystems include an antenna that communicates with the remote patient care system through radio frequency (RF) signals. Conventional antenna are relatively large, too large to fit into the housing of a leadless pacemaker.